The present invention relates to water-dispersible polyurethane polymers, and more particularly to such products synthesized from water-dispersible polyesters.
Polyurethanes find use in a variety of applications, including coatings, paints, adhesives, and the manufacture of fibers and solid articles. Polyurethanes are typically soluble in organic solvents, and exhibit little if any solubility in aqueous solvents or in systems in which water and a second water-soluble solvent are employed. Organic solvent-based resin solutions have thus typically been the vehicle of choice for use in forming coatings and the like of polyurethanes. However, many organic solvents commonly used in conjunction with such resins present environmental problems associated with their toxicity. There has been a growing concern based on the effects of such organic solvents on the environment, as well as on the long-term health of individuals who come into contact with such solvents. There has, therefore, been a growing interest in the use of aqueous resin compositions as a means of eliminating toxicity problems associated with organic solvents, and as a means of complying with governmental edicts with respect to the use of such solvents.
Examples of aqueous polyurethane dispersions are known in the art. The dispersion of the polyurethane into the aqueous medium has customarily been achieved through the use of dispersing or emulsifying agents such as nonionic or ionic surfactants. However, the use of surfactant-stabilized aqueous resin dispersions is far from desirable, since a proportion of free surfactant can be carried through to the final product, which can result in a diminution of the desired properties of the final product.
The prior art has thus attempted to incorporate directly into the polyurethane polymer molecule moieties which afford greater water dispersibility to the polymer. Typically, these attempts involve reaction of a water-soluble or water-dispersible agent such as N-(2-aminoethyl)-2-aminoethane sulfonic acid, sodium salt, or an adduct of sodium bisulfite and 1,4-dihydroxy-butene(2), as a chain extender, with the already-formed di-capped isocyanate prepolymer, thereby to form the desired polymer molecule. Typical of this approach are disclosures in U.S. Pat. Nos. 4,108,814, 4,123,423, 4,186,118, 4,306,998 and 5,528,323.
This prior art approach to the objective of incorporating a water-dispersible moiety into the polyurethane molecule has drawbacks, such as the fact that a very high content e.g. up to 70% of a solvent such as acetone needs to be present to reduce the viscosity of the mixture.
It is thus an object of the present invention to provide an improved technique for forming water-dispersible polyurethanes, particularly those formed from water-dispersible polyesters, which technique affords improved ease of operation, more satisfactory yields, and offers a greater variety in the selection of reactants and in the range of properties of the products that can be obtained.